From: Alfredo Ferrari (alfredo.ferrari@cern.ch)
Date: Thu Sep 21 2006 - 22:37:27 CEST
Dear all
I would like to stress AS MUCH AS possible the importance of what Alberto
has just pointed out. This is an extremely common misunderstanding
among High Energy Physicists who do not realize that the only meaningful
quantity is FLUENCE and NOT CURRENT.
Actually they naively believe that what they want to score is current
while they want to score fluence and they don't understand it.
In the past, in my (long) years as ATLAS radiation coordinator I had to
put a chapter in a note and explain by voice in many meetings what should
be the abc of a (radiation) physicist. I'll try to find the text and
send to the list, since it seems there are still misunderstandings
around on such basic definition.
BTW, for an isotropic beam, the error you make confusing fluence with
current is a factor 2 exact, for a monodirectional beam perpendicular
to your surface current is numerically the same as fluence, for a
monodirectional beam grazing the surface (parallel), FLUENCE is
perfectly defined, finite and equal to the one you would get with
whichever other orientation of the test surface, current is zero and your
error INFINITE!!
In essence fluence is a property of the (local) radiation
field, your test surface being a useful way to "measure" it. Current is
as dependent on the surface orientation as it is on the radiation field,
and its only purpose is to "count" charged particles (it never has any
meaning for neutral ones) in those cases where you don't care about
the amount of energy they deposit (which of course is proportional to
FLUENCE) but you operate ie in saturated regime and you get just a pulse
for each particle entering your detector, the pulse being independent of
the actual energy left by the particle (think about an RPC trigger chamber
for example)
Ciao
Alfredo
On Thu, 21 Sep 2006, Alberto Fasso' wrote:
> In the last mail of Nicole I see reported another mail, from
> E.N. Messomo, which must have been sent to her directly, because
> I have not seen it on the discussion list.
> I would like to point out a very common misunderstandig on that
> mail.
>
>>> I also started using fluence but found out that
>>> with FLUKA, fluence isn't the time integral of flux, but is rather
>>> related to the normal of the surface/boundary where the detector is
>>> defined.
>
> There is nothing strange in the way FLUKA intends fluence: it is just
> what is defined by the International Commission on Radiation Units
> and Measurements (ICRU), in its Report 60 (1998).
> On the other hand, "flux" is commonly (mis-)used in many different ways.
> Even ICRU defines it, but in a way which I have never seen used in practice:
> it is dN/dt, where N=number of particles and t=time, without any reference
> to any surface or boundary crossing.
> So, in any case "fluence" is never the "integral of flux", but the
> integral of "fluence rate". The definition by ICRU is the following
> (capitals are mine):
> ------------------------ Definition ---------------------------------------
> "The fluence, Phi, is the quotient of dN/da, where dN is the number
> of particles incident on a sphere of cross-sectional area da, thus
> Phi = dN/da.
> The use of a sphere of cross sectional area da expresses in the simplest
> manner the fact that one considers an area da PERPENDICULAR TO THE DIRECTION
> OF EACH PARTICLE. The quantities fluence and energy fluence are applicable in
> the COMMON SITUATION IN WHICH RADIATION INTERACTIONS ARE INDEPENDENT OF THE
> DIRECTION".
> ---------------------------------------------------------------------------
> Relating fluence to the normal of the boundary is a way to implement
> the above definition. If a particle crosses a surface at an angle theta,
> the area "da" to consider is not a small element da' of that surface, but
> an element of a surface perpendicular to the particle: da = da'/cos(theta).
> If this was not done, the fluence would not be independent of direction,
> as stated in the definition.
>
>>> If you simply want to score neutrons with no further
>>> information on their directions, which is what I naively assume since
>>> you have just one solid angle bin, you have to use current (FLUKA
>>> 'current' has no relationship with time). You then simply count the
>>> number of neutrons crossing a given surface.
>
> It is correct that "current" amounts to counting particles crossing
> a surface, and also that 'current' has no relationship with time.
> But fluence, too, has no relationship with time. "Information
> on particle direction" is more relevant to current than to fluence, contrary
> to what Messomo seems to think. Indeed, if you turn the surface by some
> angle, current changes but fluence does not (as stressed in the official
> definition). Scoring fluence with several angle bins will just give you
> dPhi/dOmega, i.e. the angular distribution of fluence (time integral
> of particle radiance according to ICRU).
>
>>> (Please ignore all the
>>> above if you know what you're doing and you're sure you have to use
>>> fluence...)
>
> These words don't say it explicitely, but they clearly reveal the writer's
> feeling that current is the "natural" quantity to be used in normal work, and
> fluence some exotic quantity only suited for a few very specialized
> tasks. The truth is just the opposite. Current is meaningful only
> in the rare cases where particles are counted without any interest in
> their interactions. But if one is estimating dose, activation, radiation
> damage, instrument response (all effects depending on particle interaction
> with matter), the quantity to be used is fluence and only fluence. See again
> the ICRU sentences I have written in capitals above.
>
> I would like to conclude this little tutorial on quantities by reporting a
> small note that ICRU adds to the definition of fluence.
> ---------------- Alternative definition -------------------------------------
> "In dosimetric calculations, fluence is frequently expressed in terms of the
> lengths of the particle trajectories. It can be shown that the fluence, Phi, is
> given by
> Phi = dl/dV,
> where dl is the sum of the particle trajectory lengths in the volume dV".
> -----------------------------------------------------------------------------
> This alternative definition, which I prefer by far because it gives a very
> good insight in the physical meaning of fluence (and it is not true that it is
> used only in dosimetric calculations!) is implemented in FLUKA as a
> track-length estimator (USRTRACK and USRBIN).
> The insight is the following: the number of interactions is proportional
> to the total distance travelled by the particles, NOT to the number of
> particles, NOR to the number of particles crossing a surface. This becomes
> more obvious if you measure distances in units of mean free paths.
> Coming back to the USRBDX estimator, if you think the boundary as having
> an infinitesimal thickness, the total path travelled "inside" that
> thickness depends on the cosine of the angle to the normal. Current gives
> just a number with little physical meaning.
>
> Alberto
>
>
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